Separation of platinum group metals

ABSTRACT

A process for the chromatographic interseparation of rhodium in admixture with at least one base metal from a feed solution comprising complexes of these metals. The process comprises contacting the solution with at least one chromatographic medium, and eluting one or more fractions such that at least one fraction comprises rhodium complex substantially free from base metal contamination. The at least one chromatography medium comprises a support functionalised with substituted amine groups which further comprise at least one hydrophilic group. Other platinum group metals can also be separated, both from one another and from base metals.

This invention concerns a process and a medium for the separation ofmetals in solution by chromatography, particularly for the separation ofplatinum group metals from one another and from base metals.

Previously, it has been proposed to use gel chromatography to separateplatinum group metals (PGM) from one another on an industrial scale.Prior proposals include U.S. Pat. No. 4,855,143 (Schmuckler). Thispatent describes a method in which the interseparation of platinum groupmetals (PGM) from an oxidised gold-free halide solution is achievedusing a chromatographic medium such as a polysaccharide gel (Sephadex)or a polyacrylamide gel (Biogel). The PGM when dissolved in a chloridesolution are absorbed onto the chromatographic column and are claimed tobe selectively eluted in the order ruthenium, rhodium, palladium,platinum, iridium and caesium, although it is clear from the rest of thepatent that Schmuckler meant osmium rather than caesium. The problemwith this method is that there is in fact no clear separation of PGM.

This problem was to a large extent overcome by European patentapplication EP 756013 (Matthey Rustenburg Refiners Pty) which describesa method for the interseparation of PGM from a PGM-containing halidesolution comprising the steps of passing the solution through a glycolmethacrylate chromatographic medium, absorbing the PGM onto the medium,eluting each PGM using an acid solution to obtain each fractioncontaining at least one PGM. During the elution of a reduced, mixedrhodium, iridium, ruthenium, palladium, platinum and osmium/6 molarhydrochloric acid solution through Toyopearl HWAOC using a (usually 6 M)hydrochloric acid eluent, the first eluted band contains trivalentrhodium, iridium and ruthenium. It is clear that the method does notseparate rhodium, iridium and ruthenium; either from each other or anycombinations/permutations within. Rhodium, iridium and ruthenium aresometimes referred to as the insoluble metals. Furthermore, rhodium isconventionally difficult to chromatographically separate from a mixedsolution with one or more base metals such as nickel and copper.

EP 1167555 A1 describes a process using continuous annularchromatography for the interseparation of base metals and platinum groupmetals. Two distinct media are required. The first comprises a cationexchange resin which is used to absorb the base metals, and the secondis a size exclusion gel which is used to effect a separation of the PGM.The process is further complicated by the need for an inert layer to bepresent between the two media in order to prevent intercontamination.The purity of the PGM fractions obtained is also influenced by theconcentration of the eluent and of the feed solution. Furthermore, theprocess does not separate ruthenium from rhodium.

U.S. Pat. No. 5,120,443 describes a process for the separation ofrhodium and ruthenium from iridium in a solution containing rhodium,ruthenium and iridium solution using polyamine ligands bonded toinorganic supports.

The present invention sets out to address the problem of the separationrhodium from base metals, from the other insoluble metals, ruthenium andiridium, and from other PGM using chromatography. This is of particularimportance, allowing the separation of all of the platinum group metals,both from one another and from base metals, by chromatography.Presently, refining processes involve solvent extraction, distillationand ion exchange. The metals are processed sequentially, for example inthe order osmium, gold, palladium, platinum, ruthenium, iridium andrhodium. The method of the present invention has several advantages overthe previously described methods in allowing simultaneous separation ofsome or all of these metals using a single chromatographic medium. Inaddition, the process is extremely fast, and the purity and yield of theextracted metals is high.

In accordance with the present invention, a process for thechromatographic interseparation of rhodium in admixture with rutheniumand at least one base metal from a feed solution comprising complexes ofthese metals, the process comprising contacting the solution with atleast one chromatographic medium, the steps of ensuring that theruthenium is present as a nitrosyl complex prior to, or whilst incontact with the medium and eluting one or more fractions such that atleast one fraction comprises rhodium complex substantially free frombase metal contamination and at least one fraction comprisessubstantially pure ruthenium complex; wherein the at least onechromatography medium comprises a support functionalised withsubstituted monoamine groups; and wherein the monoamine groups furthercomprise at least one hydrophilic group.

Facile recovery of rhodium is thus possible. The base metals maythemselves be chromatographically separated, the extent of such aseparation being dependent on the medium used. Although the commercialvalue of separating base metals from one another is low in comparison tothe value of recovering substantially pure rhodium, it may neverthelessbe desirable in some applications. The base metals may comprise anymetals commonly found in combination with PGM solutions for example,copper, nickel, arsenic, cobalt and iron.

Preferably, an eluent with a relatively low concentration of chlorideions is used as a first eluent in order to first elute the base metals,followed by a second eluent with a higher chloride ion concentration toelute rhodium. 1M hydrochloric acid is suitable as a first eluent, andhydrochloric acid of between 3M and 6M is suitable as a second eluent.Alternatives to hydrochloric acid include solutions of sodium chlorideor lithium chloride.

The feed solution is suitably one which has an appreciable concentrationof chloride ions, such that metals present can exist aschloro-complexes. As is common in the field of platinum group metalrefining, a hydrochloric acid solution is preferred. The concentrationof chloride ions in the feed solution may be 1M or less, but ispreferably higher than 1M, for example between 1 and 6M, and mostpreferably, 6M. Alternatives to hydrochloric acid include solutions ofsodium chloride or lithium chloride.

Preferably, the at least one hydrophilic group comprises a group chosenfrom alcohol, ether, carboxylic acid, ester, sulphone and amide, any ofwhich may be branched or unbranched, substituted or unsubstituted.

Preferably, the at least one medium comprises the following structuralformula;

wherein, R¹ is a single bond or a group chosen from alkyl, aryl,alkyl-aryl, alcohol, ether, carboxylic acid, ester, sulphone and amide,any of which may be branched or unbranched, substituted orunsubstituted; and wherein R² and R³ may be the same or different, ortogether form a cyclic group, and are chosen from the group comprisinghydrogen, alkyl, aryl, alkyl-aryl, alcohol, ether, carboxylic acid,ester, sulphone and amide, any of which may be branched or unbranched,substituted or unsubstituted, provided that at least one of R¹, R² andR³ comprises a group chosen from alcohol, ether, carboxylic acid, ester,sulphone and amide.

In a particularly preferred embodiment of the present invention, the atleast one medium comprises one or more of the following structuralformulae;

-   -   [support]-CH(OH)—CH₂—NH(CH₂—CH₂OH)    -   [support]-CH(OH)—CH₂—N(CH₂-CH₂OH)₂    -   [support]-CH(OH)—CH₂NH—C(CH₂OH)₃    -   [support]-CH(OH)—CH₂—NCH₃(CH₂—CH₂OH)    -   [support]-CH(OH)—CH₂—NH(CH₂—CH—O—CH₂—CH₂OH)    -   [support]-CH(OH)—CH₂—N(CH₂—CH₂—O—CH₃)    -   [support]-C(O)O—CH₂—CH(OH)—CH₂—N(CH₂—CH₂OH)₂    -   [support]-CH(OH)—CH₂—NC₄H₄O

Preferably, the support comprises a polymer support.

Suitably, the polymer support comprises methacrylate moieties, styrenemoieties, poly(ethylene glycol) moieties, acrylamide moieties or anycombination or mixture thereof.

The polymer support may be pre-formed or may be synthesised during theformation of the medium, and may be cross-linked to a lesser or greaterextent. Low levels of cross-linking, of the order of 1-10%, produce agel-type product, whereas higher levels of cross-linking, e.g. of theorder of 30-70% or even higher, used in combination with a suitableporogen (pore former) produce a rigid material. Porogens are known inthe art and include species such as water and xylene. Such rigidmaterials do not exhibit gel-type swelling in aqueous media but, willreadily sorb aqueous solutions into the permanent pore structure withoutsignificant volume swelling.

Particularly preferred is a co-polymer of ethylene glycol andmethacrylic acid, for example a material from the Macro-Prep (trademarkof Bio-Rad Laboratories) range of chromatographic media. This materialhas advantages in the scaling up of the chromatographic process becausehigh pressure can be applied to a column containing the material toachieve high flow rates. Alternatively or in addition, the polymersupport may be a co-polymer of oligoethyleneglycol, glycidylmethacrylateand pentaerythroldimethacrylate (for example a material from theToyopearl (trademark of TosoHaas and previously known as Fractogel)range of chromatographic media).

The polymer support may be functionalised by reaction with a substitutedmonoamine in a suitable solvent using standard laboratory procedures.Some non-limiting examples of substituted monoamines includediethanolamine, ethanolamine and morpholine. It is desirable that thefunctionalised support be washed prior to use to remove any unreactedmonoamine. Other methods and synthetic routes will be known to theskilled man for example, the support may be reacted with anun-substituted amine which is then substituted via a further syntheticstep.

The medium may be in any solid form, however preferably the medium is inthe form of a gel or a porous solid. Beads, films and fibres arepreferred. Beads may be of any particle size however preferably theyhave a mean particle size between 30 and 180 μm, more preferably between40 and 100 μm.

Alternatively, the support may be an inorganic support such as silica,alumina or zirconia.

Ruthenium is unique amongst the platinum group metals in that it formsextremely stable nitrosyl complexes (containing the NO⁺ moiety) andindeed has a well documented nitrosyl literature (Coord. Chem. rev(1978) 26 (1), 7-32, Mercer et al. Inorganic Chemistry, Vol. 3 No7, 1964pg. 1018). An extensive study has been previously performed by thepresent applicants looking at the potential use of ruthenium nitrosylspecies in precious metal refining. Of relevance in the present contextis that in 6 M HCl, there are two major ruthenium nitrosyl speciespresent: [Ru(NO)Cl₅]²⁻ and [Ru(NO)Cl₄(H₂O)]⁻. In both complexes, theruthenium is formally present in its divalent oxidation state. The[Ru(NO)Cl₅]²⁻ and [Ru(NO)Cl₄(H₂O)]⁻ species are in equilibrium, theequilibrium ratio at 6 M chloride concentration being approximately 2:1.As the chloride concentration decreases, the relative amount of the[Ru(NO)Cl₄(H₂O)]⁻ species increases and as the chloride concentrationincreases, the relative amount of the [Ru(NO)Cl₅]²⁻ species increases.

There are a number of literature methods (GB 2293372, Matthey RustenburgRefiners PTY, Spec Pub. Royal Soc. Chem. (1993), 122) for preparingruthenium nitrosyl species including the use of sodium nitrite, nitricoxide gas and nitric acid with a reductant. A presently preferred methodfor ensuring that ruthenium is present as a nitrosyl species is bytreating the feed solution with a mixture of formic acid and nitricacid.

6M hydrochloric acid is suitable for use as an eluent to recoverruthenium. It is convenient that this eluent may be the same as thatused to recover any rhodium present. Furthermore, it is a particularadvantage of the present invention that ruthenium remains on the mediumlonger than rhodium such that their separation from one another, andalso from base metals, is complete.

In a further embodiment, the feed solution further comprises a complexof iridium, and the process further comprises the steps of ensuring thatthe iridium is present in the tetravalent oxidation state whilst incontact with the medium, subsequently reducing the iridium to thetrivalent oxidation state, and eluting at least one fraction comprisingsubstantially pure iridium complex.

The feed solution may be treated to convert the iridium to thetetravalent oxidation state either prior to contacting the solution withthe medium or alternatively, iridium may be oxidised whilst in contactwith the medium. The oxidation of trivalent iridium to tetravalentiridium can be achieved using any suitable oxidising agent for example,hydrogen peroxide or chlorine.

Tetravalent iridium is strongly bound to the medium and as such needs tobe reduced to the trivalent state in order for it to be eluted andrecovered. Preferably, reduction of tetravalent iridium to trivalentiridium is effected using a mixture of hydrochloric acid and ascorbicacid, although other reducing species may be used. Once in the trivalentoxidation state, iridium can then be eluted. 6M hydrochloric acid isagain suitable as an eluent. By delaying the reduction step until afterelution of the base metals and any rhodium and ruthenium has beencompleted, the process of the invention allows the recovery ofsubstantially pure iridium.

In a yet further embodiment, the feed solution further comprises acomplex of platinum, and the process further comprises the steps ofensuring that the platinum is present in the tetravalent oxidation statewhilst in contact with the medium, and eluting at least one fractioncomprising substantially pure platinum complex.

The feed solution may be treated to ensure that the platinum is in thetetravalent oxidation state either prior to contacting the solution withthe medium or alternatively, whilst in contact with the medium. Theoxidation of divalent platinum to tetravalent platinum can be achievedusing any suitable oxidising agent. Conveniently, the same oxidisingagent used in relation to any iridium present can be used i.e., hydrogenperoxide or chlorine.

Tetravalent platinum is also strongly bound to the medium. Furthermore,it cannot easily be reduced to the divalent oxidation state and elutedusing the steps suitable for the recovery of iridium. Preferably,platinum is removed by eluting with a base. A solution of an alkalimetal or alkaline earth metal hydroxide, carbonate, or bicarbonate issuitable for example, a 1M solution of sodium hydroxide. Other basicsolutions may also be used, although ammonia is not suitable due to itforming an insoluble salt with platinum, (NH₄)₂[PtCl₆].

In a still further embodiment, the feed solution further comprises acomplex of palladium, and the process further comprises the step ofremoving the palladium from the solution prior to contacting thesolution with the medium.

Alternatively, the process further comprises the step of eluting atleast one fraction comprising palladium complex.

Palladium can be eluted using 6M hydrochloric acid however, when presentin admixture with a ruthenium nitrosyl species, both palladium andruthenium are eluted together. This presents little practical difficultyas the separation of palladium and ruthenium from an essentially binarysolution is facile and techniques for achieving this will be known tothe skilled man. Of course, in the case of a feed solution which doesnot contain ruthenium complex, essentially pure palladium complex can beobtained without further processing and no prior removal of palladiumfrom the feed is required. Methods for removing palladium from the feedsolution will also be known to the skilled man and include for example,solvent extraction using dihexyl sulphide or oximes.

The use of a base as an eluent in platinum recovery forms a furtheraspect of the present invention and accordingly, a process for thechromatographic interseparation of platinum in admixture with rhodiumand at least one base metal from a feed solution comprising complexes ofthese metals, the process comprising contacting the solution with atleast one chromatographic medium, and eluting one or more fractions suchthat at least one fraction comprises platinum complex substantially freefrom base metal contamination; wherein the at least one chromatographymedium comprises a support functionalised with substituted monoaminegroups; wherein the monoamine groups further comprise at least onehydrophilic group; and wherein platinum complex is eluted using a base.

The process may be carried out using known chromatographic techniques. Asuitable technique is batch chromatography whereby an aliquot of feedsolution is loaded onto a column comprising a chromatography medium, andeluted. A valve arrangement is employed such that the output can beswitched so that various species are collected in separate fractions. Analternative suitable method is continuous annular chromatography. Thesemethods and others will be known to those skilled in the art.

The invention will now be described by way of example only and withreference to the following drawings, in which;

FIG. 1 is a chromatogram showing the separation of a mixed PGM and basemetal solution using a commercially obtained medium;

FIG. 2 is a chromatogram showing the separation of rhodium from a mixedsolution containing base metals using an example of a process accordingto the present invention and an eluent comprising 6M hydrochloric acid;

FIG. 3 is a chromatogram showing the separation of rhodium from a mixedsolution containing base metals as in FIG. 2, but with an eluentcomprising 4M hydrochloric acid;

FIG. 4 is a chromatogram showing the separation of rhodium from a mixedsolution containing base metals as in FIG. 2, but with an eluentcomprising 3M hydrochloric acid;

FIG. 5 is a chromatogram showing the separation of rhodium from a mixedsolution containing base metals as in FIG. 2, but with a first eluentcomprising 1M hydrochloric acid, followed by a second eluent comprising6M hydrochloric acid;

FIG. 6 is a chromatogram showing an example of the separation ofrhodium, ruthenium, iridium and platinum from one another and from thebase metals copper and nickel in a mixed solution; and,

FIG. 7 is a chromatogram showing a further example of the separation ofrhodium, ruthenium, iridium and platinum from one another and from thebase metals copper and nickel in a mixed solution.

COMPARATIVE EXAMPLE

A commercial medium, Toyopearl HW-40C, with a mean particle size of 64μm, was used to pack a column of length 200 mm and diameter 5 mm. 0.25ml of a sample solution containing 20 g/l Ru, 16 g/l Rh, 5 g/l Ir, 70g/l Pd, 107 g/l Pt, 5 g/l Os and 30 g/l Ni in 6M hydrochloric acid wasthen added to the column and eluted using 6M hydrochloric acid at a flowrate of 0.375 ml/min. The results are shown in FIG. 1. It can be seenthat although a separation of Pd, Pt and Os was achieved, the insolublemetals, Ru, Rh and Ir were eluted as a single peak. Furthermore, theinsoluble metals are eluted along with Ni. This example confirms thatthis commercially available medium is not suitable for theinterseparation of the insoluble metals, either from one another or frombase metals.

Some examples of media suitable for use in the process of the presentinvention are shown in table 1 below. TABLE 1 Media code No. StructureRG004

RG006

SC001

RG012

RG018

RG023

RG020

RG028

RG010

Example 1 Synthesis of RG028

Medium RG028 was prepared by the derivatisation of MacroPrep epoxideobtained from BioRad Laboratories Ltd., Hemel Hempstead, UK. This is amacroporous bead material produced by the suspension polymerisation ofglycidyl methacrylate and ethyleneglycol dimethacrylate (approx. ratio40:60) containing 4 meq/g of epoxide groups. The manufactures quote amean bead diameter of 50 μm and a mean pore size of 1000 Å.

MacroPrep epoxide (25 g) was placed in a round bottomed flask. To thiswas added diethanolamine (15.77 g=1.1× stoichiometric excess) dissolvedin 1,4-dioxane (300 ml). The mixture was then heated to reflux withstirring. Sampling at regular intervals indicated that the reaction wassubstantially complete after seven hours. Once cooled the beads werefiltered off and soxhletted for three hours to remove any unreactedamine. The beads were then washed with acetone (3×150 ml) and driedovernight in a vacuum oven. Analysis of the product gave 2.74 meq amineper g.

Example 2 Synthesis of RG018

Medium RG018 was prepared under identical conditions to RG028 butdiethanolamine was substituted with morpholine (13.07 g). Analysis ofthe product gave 2.73 meq amine per g.

Example 3

Medium RG028, with a mean particle size of 50 μm, was used to pack acolumn of length 298 mm and diameter 10 mm. 1 ml of a sample solutioncontaining 46 g/l Rh, 45 g/l Ni, 53 g/l Cu, and 1 g/l As in 6Mhydrochloric acid was then added to the column and eluted using 6Mhydrochloric acid at a flow rate of 1.44 ml/min. The results are shownin FIG. 2. Clear separation of Rh from both Ni and Cu is achieved,although the Rh peak is partially contaminated by As and occursintermediate the base metal peaks.

Example 4

Example 3 was repeated using an eluent of 4M hydrochloric acid. Theresults are shown in FIG. 3. Broadening of the Rh peak is observedhowever, clear separation of Rh from both Ni and Cu is maintained.Furthermore, the Rh peak now occurs after both of the base metal peaks.Contamination by As is removed.

Example 5

Example 3 was repeated using an eluent of 3M hydrochloric acid. Theresults are shown in FIG. 4. Further broadening of the Rh peak isobserved however, it is shifted further from the base metal peaksallowing more facile separation. Contamination by As is removed.

Example 6

Example 3 was repeated using a first eluent of 1M hydrochloric acid for10 minutes, followed by a second eluent of 6M hydrochloric acid. Theresults are shown in FIG. 5. Sharpening of the Rh peak is observed,whilst clear separation of Rh from both Ni and Cu is maintained.Contamination by As is removed.

Example 7 Preparation of Feed Solution

A PGM refinery liquor (238 ml) containing 20 g/l Rh, 24.4 g/l Ru, 5.8g/l Ir, 152.6 g/l Pt, 0.8×10⁻³ g/l Pd, 55.5 g/l Ni, 17.3 g/l Cu and 0.7g/l As in hydrochloric acid (2.98M) was evaporated to 100 ml and thenback diluted with water to decrease the acidity to 1.44M. Conversion ofruthenium to ruthenium nitrosyl complex was achieved as follows. Formicacid (8 ml) was added at ambient temperature and the solution heated toreflux. Nitric acid (6 ml) was then added over a period of one hour andreflux continued for a further one and a half hours. The solution wasallowed to cool to ca. 40° C. before the addition of concentratedhydrochloric acid (240 ml). The solution was then refluxed for two hoursbefore being boiled back down to 240 ml.

A portion of the solution obtained above (200 ml) was then treated asfollows to oxidise trivalent iridium to tetravalent iridium. Hydrogenperoxide (22 ml, 100 vols.) was added over a period of 24 hours whilstmaintaining a gentle reflux.

After cooling, the solution was adjusted to give the final feed solutionwith a hydrochloric acid concentration of 5.14M.

Example 8

A column of length 250 mm and diameter 5 mm was packed with medium RG028to a bed depth of 202 mm. The bottom of the column was then connected tothe nebuliser of an ICP-OES spectrophotometer. 1M HCl was pumped throughthe column and the flow rate adjusted to 0.375 ml/min. A sample of thefeed solution as prepared in Example 7 was injected and elution with 1MHCl continued. Results are shown in FIG. 6. After about 10 minutes, thebase metals Cu and Ni were eluted off the column. The eluent was thenchanged to 6M HCl and elution continued for a further 77 minutes. Rh waseluted off the column first followed by Ru. The two peaks for Rucorrespond to the two ruthenium nitrosyl complexes. Tetravalent iridiumwas then reduced to trivalent iridium on the column by introducing 1%w/v ascorbic acid in 1M HCl for 5 minutes. Trivalent iridium was theneluted off the column using 6M HCl for 12 minutes. Finally, tetravalentplatinum was eluted off the column using an eluent of 1M sodiumhydroxide solution. The column was then flushed with 1M hydrochloricacid in preparation for the introduction of a further sample.

Example 9

A column of length 250 mm and diameter 5 mm was packed with medium RG018to a bed depth of 208 mm. Example 8 was repeated however, strongerretention of the complexes by the medium necessitated longer elutionperiods. These were as follows; Eluent Duration 1 M HCl   0-10 minutes 6M HCl  10-160 minutes 1% ascorbic acid in 1 M HCl 160-165 minutes 6 MHCl 165-180 minutes 1 M sodium hydroxide 180-200 minutesResults are shown in FIG. 7.

1. A process for the chromatographic interseparation of rhodium inadmixture with ruthenium and at least one base metal from a feedsolution comprising complexes of these metals, the process comprisingcontacting the solution with at least one chromatographic medium,ensuring that the ruthenium is present as a nitrosyl complex prior to,or whilst in contact with the medium and eluting one or more fractionssuch that at least one fraction comprises rhodium complex substantiallyfree from base metal contamination and at least one fraction comprisessubstantially pure ruthenium complex, wherein the at least onechromatography medium comprises a support functionalised withsubstituted monoamine groups, and wherein the monoamine groups furthercomprise at least one hydrophilic group.
 2. A process according to claim1, wherein the at least one hydrophilic group comprises a group chosenfrom alcohol, ether, carboxylic acid, ester, sulphone and amide, and ofwhich may be branched or unbranched, substituted or unsubstituted.
 3. Aprocess according to claim 1, wherein the at least one medium furthercomprises the following structural formula;

wherein, R¹ is a single bond or a group chosen from alkyl, aryl,alkyl-aryl, alcohol, ether, carboxylic acid, ester, sulphone and amide,any of which may be branched or unbranched, substituted orunsubstituted; and wherein R² and R³ may be the same or different, ortogether form a cyclic group, and are chosen from the group comprisinghydrogen, alkyl, aryl, alkyl-aryl, alcohol, ether, carboxylic acid,ester, sulphone and amide, any of which may be branched or unbranched,substituted or unsubstituted, provided that at least one of R¹, R² andR³ comprises a group chosen from alcohol, ether, carboxylic acid, ester,sulphone and amide.
 4. A process according to claim 1, wherein thesupport comprises a polymer selected from the group consisting ofmethacrylate moieties, styrene moieties, poly(ethylene glycol) moieties,acrylamide moieties and any combination or mixture thereof.
 5. A processaccording to any of claim 1, wherein the support comprises an inorganicsupport selected from the group consisting of silica, alumina andzirconia.
 6. A process according to claim 1, wherein the at least onemedium is in the form of a gel, porous solid or fibre.
 7. A processaccording to claim 1, wherein the feed solution further comprises acomplex of iridium, and wherein the process further comprises the stepsof ensuring that the iridium is present in a tetravalent oxidation statewhilst in contact with the medium; subsequently reducing the iridium toa trivalent oxidation state; and eluting at least one fractioncomprising substantially pure iridium complex.
 8. A process according toclaim 1, wherein the feed solution further comprises a complex ofplatinum, and wherein the process further comprises the steps ofensuring that the platinum is present in a tetravalent oxidation statewhilst in contact with the medium; and eluting at least one fractioncomprising substantially pure platinum complex.
 9. A process accordingto claim 1, wherein the feed solution further comprises a complex ofpalladium, and wherein the process further comprises the step ofremoving the palladium from the solution prior to contacting thesolution with the medium.
 10. A process according to claim 1, whereinthe feed solution further comprises a complex of palladium, and whereinthe process further comprises the step of eluting at least one fractioncomprising palladium complex.
 11. A process for the chromatographicinterseparation of platinum in admixture with rhodium and at least onebase metal from a feed solution comprising complexes of these metals,the process comprising contacting the solution with at least onechromatographic medium; and eluting one or more fractions such that atleast one fraction comprises platinum complex substantially free frombase metal contamination, wherein the at least one chromatography mediumcomprises a support functionalised with substituted monoamine groups,wherein the monoamine groups comprise at least one hydrophilic group,and wherein platinum complex is eluted using a base.
 12. A processaccording to claim 11, wherein the base comprises an alkali metal oralkaline earth metal hydroxide, and a carbonate or bicarbonate.